Graphite is utilized in many forms in the chemical and metallurgical industries for high temperature applications. Typical of these applications are for crucibles used in the melting of many metals, for electrodes in electro-arc furnaces, etc. Graphite crucibles or molds, for example, exhibit good mechanical strength and excellent thermal stability under the temperature conditions encountered. These same attributes are important in the electrodes.
A major deficiency to the use of graphite and other carbonaceous articles is that they rapidly oxidize and erode at temperatures above about 500 degrees C. since the carbon reacts with oxygen to form CO and/or CO.sub.2. Thus, in the case of the crucibles, they become thinned and periodically must be replaced. Graphite rods, when used as electrodes, "pencil down" at the end and continue to erode. While some consumption is expected in the melting process, the oxidation loss accelerates the erosion/corrosion and reduces the current-carrying capacity. Accordingly, the electrode must be progessively advanced into the melt and then additional lengths must be added.
A number of coating materials have been developed to reduce the problem of oxidation. Typical of these coatings are described in U.S. Pat. No. 2,995,453, issued to R. D. Noble, et al., on Aug. 8, 1961; U.S. Pat. No. 4,559,270, issued to R. V. Sara on Dec. 17, 1985; U.S. Pat. No. 4,418,097, issued to M. S. Misra on Nov. 29, 1983; U.S. Pat. No. 3,140,193, issued to J. S. Kane on July 7, 1964; U.S. Pat. No. 4,301,387, issued to J. Schiffarth, et al., on Nov. 17, 1981; U.S. Pat. No. 3,348,929, issued to A. J. Valtschev, et al., on Oct. 24, 1967; and U.S. Pat. No. 2,749,254, issued to J. A. Slyh, et al., on June 5, 1956. Many of these coatings must be applied under conditions of elevated temperature under an inert atmosphere in order to achieve a highly adherent coating. These processes, due to the extremes of conditions, can result in gaps of the coating leaving those areas to be subjected to oxidation. In the '270 patent, for example, heating to at least 1100 degrees C. is required; in '254, a temperature of about 2200 degrees C. is required. The coating of '929 requires "metallization", and in '387 the coating is applied in the form of pre-formed sheet material.
The coatings of the prior art contain various forms of silicon compounds and alumina compounds: silicon carbide, sodium silicate, mullite being typical. Others have varying proportions of boron, phosphates, etc., including many of the refractory oxides. Although some of these compounds are inexpensive, others are not. Furthermore, many of the coatings derived from silicates tend to be hydroscopic (collect moisture from the atmosphere on the surface). This excessive moisture can be detrimental when the coated object comes in contact with molten metal, for example. In addition, these coatings are generally so electrically insulative that they cannot be used on electrodes proximate the current clamps. At least a portion of the shortcomings of the prior art coatings is due to an inadequate binder/suspension agent for "protective" element of the coating composition.
Accordingly, it is one object of the present invention to provide a material for effecting a corrosion resistant coating on graphite and similar carbonaceous bodies.
It is another object to provide a material that forms an effective protection against oxidation of graphite and like materials that does not require "conditioning" at elevated temperatures.
Another object is to provide a "paint-like" material that can be applied to graphite and like materials and then dried at near room temperature to achieve an oxidation prevention coating.
Still another object is to provide a method for producing a "paint-like" material having long shelf life that can be applied to graphite and the like materials and dried at near room temperature to produce an oxidation prevention coating.
A further object is to provide an inexpensive oxidation prevention coating for graphite and like materials that can be formulated to be either electrically conductive or non-conductive.
It is also an object of the present invention to provide a binder/suspension agent for use with refractory compounds and the like so as to produce, for example, a coating useful to prevent oxidation of graphite.
An additional object of the present invention is to provide a method of producing a binder/suspension agent for use with refractory compounds and the like.
These and other objects of the present invention will become apparent upon a consideration of the following drawing and a complete description which follow.